There has been increasing interest in coincident phase center elements for electronically steered, polarization diverse, ultra wideband array antennas in recent years. This interest has arisen from the difficulty of maintaining the axial ratio of circularly polarized beams when scanning off-axis. When the constituent linear polarizations used to form circular polarization have adjacent phase centers, an angle dependant path length difference is introduced upon scanning. With increasing bandwidth, compensating for this path difference becomes more difficult. The motivation for developing ultra wideband coincident phase center antennas is to eliminate the scan dependant path length difference associated with adjacent phase center antennas.
Navy ships require electronically steerable antenna arrays capable of transmitting and receiving signals with polarization diversity, including circularly polarized waves, over large instantaneous bandwidth for satellite communications, electronic warfare, and other applications. Antenna element designs include those for operation using different polarizations, including linear (vertical or horizontal) and circular. These antenna elements are typically assembled into arrays for generating or receiving a collimated, directed RF beams.
To obtain polarization diversity, an antenna needs to radiate two orthogonal polarizations independently. This can be done with a pair of orthogonally positioned, linearly polarized elements. If electronic steering of circularly polarized beams is desired, then the linearly polarized elements must also have coincident phase centers to avoid degradation of circularity as the beam is scanned. The polarization purity degrades further in the case of wide instantaneous bandwidth signals.
Ultra wideband antenna arrays frequently employ flared notch radiators. This is because flared notch radiators usually do not have a strong resonance, but rather may be viewed as smooth tapers from a confined transmission line mode to a radiating free space mode. The difficulty with flared notches is that the individual radiators require conductive contact between adjacent elements to operate correctly at low frequencies within their design bandwidths
Employing elements which do not require conductive contract between adjacent elements may free the designer from the difficulties of maintaining electrical contact between adjacent elements, but may introduce problem of obtaining large bandwidths from elements not known for wide bandwidth.
With regard to bandwidth enhancement, there are two frequency regimes to consider: the high frequency regime where in a half wavelength is less than the array cell size and the low frequency regime where in a half wavelength greater than the array cell size. The low frequency regime is more interesting for electronic beam steering applications while the high frequency regime is more interesting for fixed scan or broadside applications. In the low frequency regime a two to one bandwidth is readily attainable while maintaining simple construction methods. A four to one bandwidth is achievable with special construction techniques. The limits of the high frequency regime are encountered when interference between direct radiation from the dipoles, and reflected radiation from the ground plane behind the dipoles begins to form a null in the radiation pattern.
There are some trade off between low and high frequency performance. The more the bandwidth is extended in the low frequency regime, the greater the reflections seen in the high frequency regime. When the special construction techniques are employed to extend the bandwidth to 4:1 in the low frequency regime, there is very little bandwidth left in the high frequency regime.
A cross sectional view of a unit cell 10 in a coincident phase center array antenna with a coincident phase center flared notch element is shown in FIG. 1. This antenna is difficult to build because it must be constructed to maintain electrical continuity in multiple places for two interleaved perpendicular polarizations simultaneously. The first polarization is in the plane 12 of the sectional cut, and has its feed circuitry 14 exposed. The second polarization plane 16 is perpendicular to the section plane. Within a unit cell, it is important to maintain microwave electrical continuity across the slot 18 (further described below in an E-plane tee configuration). Between unit cells it is important to maintain microwave electrical continuity at the flare tips 20. Here microwave continuity implies control of the geometry of mating conductors so that the smooth flow of microwave fields can occur. Direct current continuity simply requires that mating conductors be in contact. The double balun dipole according this intention does not require continuity between unit cells in an array which greatly simplifies construction.
The next design to be considered is not a coincident phase center antenna, but it is introduced to help explain later designs. FIG. 2 shows a microstrip fed printed dipole with an integral balun by Edward and Rees, as described in U.S. Pat. No. 4,825,220, issued Apr. 25, 1989. It has a single Marchand balun 50 feeding the two arms of the dipole. It has a bandwidth 3 or 4 times greater than a traditional split sleeve dipole and it is fabricated using printed circuit techniques.
Before discussing the next patents/prior art, it will be useful to address microwave transmission lines. This will help to understand the shortcomings of the patents/prior art to be discussed. The classic transmission line is the parallel wire transmission line 60 shown in FIG. 3. The two conductors 62 are the same size, and the currents which flow on them are mirror images of one another. The difficulty with parallel wire transmission lines is that they must be protected from the rest of the world. A conductor that is allowed too close can cause reflections or permit extra modes to propagate, and dielectrics can change the characteristic impedance or introduce losses. A slot 64 is also shown in FIG. 3 as an example of balanced transmission line. Unless the conductors 66 and 68 on each side of the slot have the same cross-section, the transmission line mode it supports is only approximately balanced. In general slots tend to radiate, so it is desirable to keep them short with respect to a wavelength. In practice most microwave transmission lines are unbalanced lines such as those shown in FIG. 4. One conductor 70 is made much larger than the other conductor 72, and is often grounded. It shields the smaller conductor from the surroundings, making the circuit much easier to work with. Stripline has two ground conductors, but they are often joined with screws or plated vias, so it is similar to coaxial line. Direct connections between balanced and unbalanced transmission lines generally result in undesirable large reflections. Usually a balun circuit is inserted at the junction to enable proper flow of signals across the boundary.
The groundwork has been laid to consider the Wideband Phased Array Antenna and Associated Methods by Munk, Taylor, and Durham (“Munk et al.”), as described in U.S. Pat. No. 6,512,487 and in “A Wide Band, Low Profile Array of End Loaded Dipoles with Dielectric Slab Compensation,” Ben A. Munk, 2006 Antenna Applications Symp., pp. 149-165 (“Munk”), shown in FIG. 5. In the patent Munk et al. claims a 15:1 bandwidth, while in the paper Munk shows plots indicating a 9:1 bandwidth. Both are remarkable. Through the use of interdigital capacitors at the ends of the dipole arms, and selective use of dielectric layers, Munk and Munk et. al. have constructed a dipole array which is intrinsically well matched over an extremely wide bandwidth. Having constructed well matched dipoles, they must feed them with a balanced transmission lines. However, as I pointed out above, balanced transmission lines are undesirable because they need to be shielded. Munk et al. does not describe the particulars of the feed network in the patent. However, another patent, U.S. Pat. No. 6,483,464, Rawnick et al., describes a feed network and associated feed line organizer, shown in FIG. 6, which they state is suitable for the dipole array antenna patented by Munk et al. Rawnick et al. state that the feed line organizer suppresses common mode currents. Indirectly, this may be an affirmation of the difficulty noted above with balanced transmission lines. An isolated single two conductor transmission line should support only one mode of propagation. If a second two conductor transmission line is brought into close proximity of the first, the result is a four conductor transmission line which can support unwanted modes. The self-shielding of unbalanced transmission lines reduces the coupling to nearby transmission lines. In effect, Rawnick et al. has replaced two balanced transmission lines in close proximity with four unbalanced transmission lines with the feed line organizer. A 180 degree hybrid circuit is also shown in FIG. 6. The balanced transmission line mode of Munk et al. and Munk's dipole still needs to be converted to an unbalanced mode.
Typically a 180 degree hybrid has several quarter wavelength sections. This is at some intermediate frequency, not the highest frequency. However the space available is a square one half wavelength on a side, and this is at the highest frequency. The size of the circuit can be reduced by the use of high dielectric constant materials, but only to a point. Practical circuit processing techniques limit how small features of a circuit can be made. Munk's design is very clever. However the dipole according to this invention has the advantage of providing unbalanced transmission line modes right at the terminals of the antenna. The conversion from a balanced mode to an unbalanced mode is implemented more efficiently in less space with a double Marchand balun dipole than with a balanced dipole, feed line organizer, and 180 degree hybrid. Munk notes that his design is capable of dual polarized operation, but he does not mention coincidence of phase centers. It is therefore desirable to provide a dipole antenna without these deficiencies.